The present invention relates to an optical coupling device, and particularly to an optical coupling device for coupling light, which has gone out of a light outgoing member, to a light incoming member by means of an optical lens.
Along with the recent progress of information-oriented society, the field of optical communication has been rapidly developing.
In the field of optical communication, rapid development toward higher functions such as higher transfer rate or higher multiplexing of data has been realized.
To realize widespread optical communication to ordinary homes, that is, to realize a broad band network, costs in the field of optical communication must be reduced.
Optical fibers are mainly classified into two categories: expensive glass made optical fibers allowing long-distance transmission of signals at a high transfer rate with less deterioration of signals, and inexpensive plastic made optical fibers (POF) allowing only short-distance transmission of signals.
The reason why the cost of optical fibers is high at the present time is not only due to the fact that the cost of high performance glass made optical fibers is high, but also due to the fact that the cost of mounting optical fibers is high.
FIG. 16 is a perspective view showing a configuration of a related art optical coupling device for mounting optical fibers.
An optical fiber mounting substrate 100 has recesses 101 for lenses and grooves 102 for optical fibers. A ball-shaped lens 103 is disposed in each of the recesses 101 and an optical fiber 4 is disposed in each of the grooves 102. With this configuration, optical coupling is made between an optical device plane 105 of each of the light emitting devices or light receiving devices provided on an optical device substrate 104 and the corresponding one of the optical fibers 4.
In the above-described optical coupling device, since the ball-shaped lenses 103 are used as the optical lenses, the optical fibers 4 and the optical device planes 105 can be easily positioned to each other via the ball-shaped lenses 103; however, since the ball-shaped lenses 103 are not easy to handle, the recesses 101 for lenses are required to be previously provided in the optical fiber mounting substrate 100.
The optical fiber mounting substrate 100, with the recesses 101 and the grooves 102, is to be provided with an expensive anisotropically etchable substrate, for example, a silicon substrate. The use of such an expensive substrate is one reason why the cost of mounting optical fibers is rising.
To reduce the above-described mounting cost of optical fibers, there have been known optical coupling devices using no ball-shaped lenses; however, these optical coupling devices have other problems.
FIG. 17 is a typical view showing a schematic configuration of an optical coupling device for optically coupling an optical fiber to a light emitting device such as a semiconductor laser, a light emitting diode, or a plane emission type semiconductor laser by using an optical lens formed by a diffusion method characterized by diffusing a specific element into an optical lens material.
Referring to FIG. 17, there is shown an optical coupling device configured such that a light emitting device 2a, an optical lens 1a, and an optical fiber 4 are disposed at specific positions. The light emitting device 2a includes a light emitting device substrate 20 and a light emitting portion, for example, a light emitting diode portion 22 provided on the light emitting device substrate 20. The light emitting diode portion 22 may be replaced with a semiconductor laser portion or a plane emission type semiconductor laser portion. The optical lens 1a includes a lens substrate 10 and a convex portion 11 functioning as an optical lens formed on the lens substrate 10 by the above-described diffusion method. The optical fiber 4 includes a core portion 40 and a cladding portion 41 provided around an outer peripheral portion of the core portion 40.
A light ray L, which has gone out of the light emitting diode portion 22 of the light emitting device 2a, is coupled to a light incoming portion located at an end face of the optical fiber 4 by means of the optical lens portion 11.
In the case of using the convex portion 11 formed by the diffusion method as the optical lens, since an increased rate of a refractive index of the convex portion is generally small, it is difficult to obtain an optical lens having a high numerical aperture (NA).
For example, in the case of forming a convex portion by diffusing titanium (Ti) into a substrate made from lithium niobate, since an increased rate of a refractive index is about 4%, an NA of an optical lens composed of the convex portion becomes as small as about 0.1.
Accordingly, in order to project light, which has gone out of the light emitting diode portion 22 of the light emitting device 2a, to the light incoming portion located at the end face of the optical fiber 4, as shown in FIG. 17, a distance between the light emitting diode portion 22 and the optical lens must be extended, with a result that only part of the light having gone out of the light emitting diode portion 22 can be converged. That is to say, of all of the light rays Lw shown by broken lines, only part of the light rays LW, that is, light rays L shown by solid lines can be effectively used.
In this case, since it is required to suppress crosstalk of signals caused by incidence of part of the light, which has not been converged by the optical lens, on the adjacent optical fiber, it is required to absorb such part of the light by forming a light absorbing mask AM having apertures in the optical path.
As a result, in the case of using such an optical lens having a low NA, it is impossible to effectively use light emerged from an optical fiber.
FIG. 18 is a typical view showing a schematic configuration of an optical coupling device for optically coupling an optical fiber to a light emitting device such as a semiconductor laser, a light emitting diode, or a plane emission type semiconductor laser without using an optical lens.
Referring to FIG. 18, there is shown an optical coupling device configured such that a light emitting device array 2 and a plurality of optical fibers 4a and 4b are disposed at specific positions. The light emitting device array 2 includes a light emitting device substrate 20, and a plurality of light emitting portions, for example, light emitting diode portions 22a and 22b. The light emitting diode portions may be replaced with semiconductor laser portions or plane emission type semiconductor laser portions. Each of the optical fibers 4a and 4b includes a core portion 40 and a cladding portion 41 provided around an outer peripheral portion of the core portion 40. A light ray L, which has gone out of each of the light emitting diode portions 22a and 22b of the light emitting device array 2, is coupled to a light incoming portion located at an end face of the corresponding one of the optical fibers 4a and 4b. 
In this case, since the light ray, which has gone out of each of the light emitting diodes 22a and 22b, has an extension angle, a reflected light ray LR from the end face of the corresponding one of the optical fibers 4a and 4b is possibly made incident on one or more optical fibers adjacent thereto, thereby causing crosstalk of signals, and making it difficult to enhance an utilization factor of light.
On the other hand, even in the case of disposing optical fibers on the light outgoing side, there arises a problem similar to that described above.
FIG. 19 is a typical view showing a schematic configuration of an optical coupling device for optically coupling an optical fiber to a light receiving device such as a photodiode by using an optical lens formed by the diffusion method.
Referring to FIG. 19, there is shown an optical coupling device configured such that a light receiving device 5a, an optical lens 1a, and an optical fiber 4 are disposed at specific positions. The light receiving device 5a includes a light receiving device substrate 50 and a light receiving portion such as a photodiode portion 51 provided on the light receiving device substrate 50. The optical lens 1a includes a lens substrate 10 and a convex portion 11 functioning as an optical lens formed on the lens substrate 10 by the diffusion method. The optical fiber 4 includes a core portion 40 and a cladding portion 41 provided around an outer peripheral portion of the core portion 40.
A light ray L, which has gone out of a light outgoing portion located at an end face of the optical fiber 4, is coupled to the photodiode portion 51 of the light receiving device 5a. 
As described above, since the NA of the optical lens formed by the diffusion method is small, to project light having gone out of the optical fiber 4 to the photodiode portion 51 of the light receiving portion 5a, a distance between the optical fiber 4 and the optical lens must be extended, with a result that only part of the light having gone out of the optical fiber 4 can be converged by the optical lens as shown in FIG. 19. That is to say, of all of the light rays Lw shown by broken lines, only part of the light rays LW, that is, light rays L shown by solid lines can be used. In this case, since it is required to suppress crosstalk of signals caused by incidence of part of the light, which has not been converged by the optical lens, on the adjacent optical fiber, it is required to absorb such part of the light by forming a light absorbing mask AM having apertures in an optical path.
As a result, as described above, in the case of using such an optical lens having a low NA, it is impossible to effectively use light emerged from an optical fiber.
FIG. 20 is a typical view showing a schematic configuration of an optical coupling device for optically coupling an optical fiber to a light receiving device such as a photodiode.
Referring to FIG. 20, there is shown an optical coupling device configured such that a light receiving device array 5 and a plurality of optical fibers 4a and 4b are disposed at specific positions. The light receiving device array 5 includes a light receiving device substrate 50 and a plurality of light receiving portions such as photodiode portions 51a and 51b provided on the light receiving device substrate 50. Each of the optical fibers 4a and 4b include a core portion 40 and a cladding portion 41 provided around an outer peripheral portion of the core portion 40. A light ray L, which has gone out of a light outgoing portion located at an end face of each of the optical fibers 4a and 4b, is coupled to the corresponding one of the photodiode portions 51a and 51b of the light receiving device array 5.
In this case, since the light, which has gone out of the end face of each of the optical fibers 4a and 4b, has an extension angle, a reflected light ray LR from the corresponding one of the photodiode portions 51a and 51b is possibly incident on one or more optical fibers and/or the light receiving portions adjacent thereto, thereby causing crosstalk of signals, and making it difficult to enhance an utilization factor of light.